1. Field of the Invention
The present invention relates generally to community alarm systems and more particularly to the use of DC voltage polarity reversal in subscriber telephone circuits from a central office distribution frame to activate alarms at distributed subscriber locations.
2. Description of the Prior Art
Emergency alert systems have been devised to warn citizens of dangerous conditions which may exist due to chemical or nuclear plant accidents, fires, floods or other disaster. Use of existing telephone networks for these alarm systems allows the warning authority to reach a large cross section of the affected population. Current warning systems utilize the Central Office Switch to dialup each customer to be warned. Telephone networks provide effective and cost efficient media for the transfer of signals which may be used to activate an alarm at the subscribers location.
DC control signals have been used for many years as an integral component of telephone systems. These DC signals include the presence or absence of circuit current or voltage, or of a specific voltage polarity. DC signals have been used to indicate on-hook and off-hook conditions at subscriber locations, dial pulses, or connection status as a call is initiated. For example, the circuit between a telephone handset and the central office servicing the telephone is typically open when the handset is in its on-hook position. Continuous current flows through a closed circuit when the handset is in an off-hook position. Current flow interrupted at a specified rate is interpreted as pulse dialing. DC signals are advantageous due to their simplicity. Since they either involve on/off or positive/negative choices, these signals are easily recognized by inexpensive means. Tone signalling methods are also common for status and control. The dial tone and the busy signal provide the caller with information about the status of the call being initiated. Dual Tone Multifrequency (DTMF) telephones use unique combinations of tones to send control information from a push-button telephone to the central office, typically to define the phone number which is being called. DC and tone signalling have been combined to provide extensive phone services.
Alarm systems have used DC and tone signals to receive information from subscribers, as in a burglar alarm system. U.S. Pat. No. 3,757,323 to Pintell relates to a monitoring system for which a reversible DC voltage is impressed upon the conductors of a two wire line at each of the subscriber locations. The occurrence of an event or condition which is being monitored (e.g. an indication of forced entry) causes a reversal of the polarity at the subscriber location. This DC signal is transmitted from the subscriber to a central monitoring station and is detected by a coupler at the central station. U.S. Pat. No. 3,027,424 to Breed also relates to a system involving polarity reversal to achieve different desired results. In Breed, polarity switching is used to determine whether to collect or refund coins from a pay phone. In Breed, however, the activation of a relay to collect a coin is initiated by establishing a positive circuit polarity at the pay phone in response to the depositing of a coin. The reversal of the circuit polarity to negative in order to refund a coin is initiated at the central office.
Alarm systems in which a signal is broadcast from a single location to multiple subscribers are also known. U.S. Pat. No. 3,643,029 to Breazeale teaches a subscriber receiver unit having a frequency detector for sensing the simultaneous transmission of two specific frequencies by transmitter 10 from the central office. The receiver unit is connected to the existing Tip and Ring pair which joins the subscriber telephone to the central office. The receiver unit is installed in parallel with the subscriber telephone.
The Breazeale system requires special equipment in the central office which effectively dials up each subscriber to be warned of the emergency. The use of a tone signal requires audio generators 14, 30 filter 12, and a frequency detector at each subscriber site, increasing the total hardware costs for implementing this system. In addition, this system ties up a portion of the central office switch during the emergency situation, at a time when there is likely to be an increased demand for telephone service.